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Is this a horribly ambiguous A Level Physics question?

  • Thread starter ChrisXenon
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Hi, I'm a private tutor who is not a school teacher and has no contact with schools.

I have an OCR A level "physics A" student, and we're working through past papers. Time and time again, we've come across questions which are astonishingly ambiguous and answer schemes which are in my view, astonishingly picky or just plane bizarre. I've taken some of these to OCR but got no progress.

For example they maintain that the word "rate" implies an increasing quantity, whereas I believe that "rate of pay" simply means how much you get paid. They say that "acceleration" will not do when a body is decelerating whereas I think that "deceleration" is an acceleration with a negative value and that all decelerations are accelerations.

We can't agree, but I'm not convinced I'm wrong, so I am looking for broader perspectives. If I AM wrong, then I need to know it and change my teaching accordingly. If I AM NOT wrong, then there's is something very worrying about how we measure OCR physics A level candidates.

So I'm looking for insights from teachers or physics experts.

Here is an example from yesterday's lesson. OCR Physics A H156/01 May 2016, Q27. A graph shows an I/V curve for an LED whose threshold voltage is 2.7V.

Question a: Describe and justify the variation of resistance R of the LED as the potential difference V across the LED is increased from:
(i) -1.0V to 2.6V
(ii) 2.6V to 3.0V
(iii) 3.0V to 3.4V

- but what does it mean by "and justify"?

I thought it meant "explain in terms of semiconductor behaviour", and - without prompting - my student thought the same. But in fact, the answer scheme makes clear that it wants stuff like "R is infinite because R = V/I = 2.9 / 0". OK but what is required is simply not clear from the question which in my view is horribly ambiguous. Both my student and I would have scored no points for that question. And these instances arise in every paper and sometimes more than once.

Relating to this - how do marking schemes work in practice; are they adhered to as the letter of the law or are they taken with a pinch of salt and markers go with "the right answer" as they see it. Actually either is worring but for different reasons.

So - any insights you can offer, on this question in particular, or the examples above it, or on exam querstions & marking - would be very welcome.

Thanks
 
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For example they maintain that the word "rate" implies an increasing quantity, whereas I believe that "rate of pay" simply means how much you get paid. They say that "acceleration" will not do when a body is decelerating whereas I think that "deceleration" is an acceleration with a negative value and that all decelerations are accelerations.
You are correct on both counts and I also agree with your analysis of the other problem you mentioned.

I've often found this kind of idiocy on "standardized" tests, which often seem to have been formulated by bureaucrats, not teachers.
 
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For example they maintain that the word "rate" implies an increasing quantity, whereas I believe that "rate of pay" simply means how much you get paid. They say that "acceleration" will not do when a body is decelerating whereas I think that "deceleration" is an acceleration with a negative value and that all decelerations are accelerations.
Regarding rates, they usually mean how some quantity is changing relative to some other quantity, where the "other quantity" is often time, but could be something other than time. In your example, "pay rate" is not just how much you get paid -- there is an implied time period. If my hourly pay rate is $30, this rate is actually ##\frac{$30}{\text{hour}}##.
Regarding acceleration, I agree that acceleration could be negative, zero, or positive, but if the question specifically asks whether an object is accelerating or decelerating, and a < 0, then the correct answer would be "decelerating," IMO.
 
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If my hourly pay rate is $30, this rate is actually ##\frac{$30}{\text{hour}}##.
But that's not an "increasing quantity" which is what he asked about
Regarding acceleration, I agree that acceleration could be negative, zero, or positive, but if the question specifically asks whether an object is accelerating or decelerating, and a < 0, then the correct answer would be "decelerating," IMO.
fair point.
 
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But that's not an "increasing quantity" which is what he asked about
The salary/wage paid is increasing -- at a rate of $30/hour in my example.
 
But that's not an "increasing quantity" which is what he asked about
fair point.
Guys thanks for your comments so far. Mark44 - thanks - you have helped me to understand what the OCR question-setter was saying.

In that case, the marking scheme forbade the term "rate of acceleration". In that context I still see the point but I feel it is very weak. When someone says "rate of acceleration" I take it to mean the rate of change of veolicty with time.

I think this may be a case where tying down words so specifically can impede the freedom od markers to behave intellignelty. Of course physics is a world where language must be used carefully to precisely express concepts and demonstrating understanding must be done through the written word so care is mandatory but in my view, this stipulation is wrong.

Interestingly, in the next part of that question "rate" is allowed. They felt is was unfair to penalise it's incorrect usage twice.

Actually (and rhetorically, maybe) if we think rate means change then why do we so often say "rate of change"?
 
For completeness, in case anyone's interested, here is the complete context of the "rate" question as originally sent to OCR:

A velocity/time graph shows an upward-curving line between t=0 & 2, and a straight descending line between 4=2 & 7. The question invites the student to describe the motion of the object.

That seems clear enough until you see the answer scheme. Two things puzzle me:

1. It FORBIDS any answers using "rate of acceleration". Please explain why. Later on (Q2a), you accept that same term. What is wrong with it?

2. Part 2 FORBIDS "constant acceleration". Why? The term "deceleration" is accepted, but any constant deceleration is also a constant acceleration whose magnitude is negative; it's still an acceleration. If I toss a ball into the air and consider its vertical velocity, we can note that it will have an initial upwards velocity (which we can arbitrarily label "positive"). It will slow down, then stop, then change direction and velocity will increase in the negative direction. If we plot its vertical velocity over that time, we'll see those velocities move from positive, through zero, to negative. Throughout this period the acceleration is a constant - g. So it has sped up, stopped, changed direction and sped up again, but it's perfectly correct to say it's ACCELERATION was constant, and it would be odd to say its DECELERATION was anything.
 
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The salary/wage paid is increasing -- at a rate of $30/hour in my example.
??? that's not what you said.
In your example, "pay rate" is not just how much you get paid -- there is an implied time period. If my hourly pay rate is $30, this rate is actually ##\frac{$30}{\text{hour}}##.
I think an increasing rate of pay would be when you get a raise, but I sort of see how you are looking at it. You are considering that the PAY is increasing, not that the RATE of pay is increasing.
 
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vela

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Question a: Describe and justify the variation of resistance R of the LED as the potential difference V across the LED is increased from:
(i) -1.0V to 2.6V
(ii) 2.6V to 3.0V
(iii) 3.0V to 3.4V

- but what does it mean by "and justify"?

I thought it meant "explain in terms of semiconductor behaviour", and - without prompting - my student thought the same. But in fact, the answer scheme makes clear that it wants stuff like "R is infinite because R = V/I = 2.9 / 0". OK but what is required is simply not clear from the question which in my view is horribly ambiguous. Both my student and I would have scored no points for that question. And these instances arise in every paper and sometimes more than once.
Justify means you need to explain how you got the answer. Many students may be able to tell you the resistance is effectively infinite below the threshold voltage, but if you were to ask them why it is so, they'll say, "I don't know," because all they did was memorize a result without understanding the reasoning behind it.

As to what level of justification is appropriate, that's a part of the art of test-taking. Frankly, given the question, I think your interpretation of explaining in terms of semiconductor behavior would be overkill, but you know the test better than I do.
 
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The salary/wage paid is increasing -- at a rate of $30/hour in my example.
??? that's not what you said.
I think it's exactly what I said. I am distinguishing between the pay (amount of money) and the pay rate (amount of money per unit of time).
At a pay rate of $30/hour your total amount to be paid increases by $30 each hour. In terms of graphs, the pay rate is the slope of the graph of P = 30t, or 30. The total amount paid for t hours would be 30t dollars.
Think an increasing rate of pay would be when you get a raise, but I sort of see how you are looking at it.
In my example, the rate of pay is constant (30), but the amount paid is increasing.
 

tech99

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I have always been worried about these things, but there is no easy answer.
Today we did a question asking for two advantages of a temp probe/data logger over a thermometer. Well it has better resolution, but the second answer was that there is less chance of error with a digital display. I would be hard pressed to have picked the answer they want out of several possibilities.
 
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In my example, the rate of pay is constant (30), but the amount paid is increasing.
I think you must have responded while I was editing my post because I realized that. Thanks.
 
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For completeness, in case anyone's interested, here is the complete context of the "rate" question as originally sent to OCR:

A velocity/time graph shows an upward-curving line between t=0 & 2, and a straight descending line between 4=2 & 7. The question invites the student to describe the motion of the object.
Above, I assume you meant "straight descending line between t = 2 and 7"

On the interval [0, 2] is the graph concave up? If so, we have v > 0 and increasing, and acceleration > 0. On the interval [2, 7], we have v > 0 (I think, but don't have the graph to look at) and decreasing, but if the graph shows a straight line, the acceleration < 0. I.e., the object is decelerating.
ChrisXenon said:
That seems clear enough until you see the answer scheme. Two things puzzle me:

1. It FORBIDS any answers using "rate of acceleration". Please explain why. Later on (Q2a), you accept that same term. What is wrong with it?
Because acceleration already is a rate (##\frac {dv}{dt}## or the second derivative of the position with respect to time). "Rate of acceleration" implies how the acceleration is changing with respect to t.
ChrisXenon said:
2. Part 2 FORBIDS "constant acceleration". Why? The term "deceleration" is accepted, but any constant deceleration is also a constant acceleration whose magnitude is negative; it's still an acceleration. If I toss a ball into the air and consider its vertical velocity, we can note that it will have an initial upwards velocity (which we can arbitrarily label "positive"). It will slow down, then stop, then change direction and velocity will increase in the negative direction. If we plot its vertical velocity over that time, we'll see those velocities move from positive, through zero, to negative. Throughout this period the acceleration is a constant - g. So it has sped up, stopped, changed direction and sped up again, but it's perfectly correct to say it's ACCELERATION was constant, and it would be odd to say its DECELERATION was anything.
Like I said earlier, it seems that the part 2 answer scheme wants the student to say simply whether the object is accelerating or decelerating. The simplest answer, without getting into the weeds with pos./neg. acceleration is that the object is accelerating for the first two seconds, and decelerating for the next five seconds. When humans write these questions and answers for automated-scoring tests, they don't always think of all the varied ways that other humans can respond.
 
Above, I assume you meant "straight descending line between t = 2 and 7"

On the interval [0, 2] is the graph concave up? If so, we have v > 0 and increasing, and acceleration > 0. On the interval [2, 7], we have v > 0 (I think, but don't have the graph to look at) and decreasing, but if the graph shows a straight line, the acceleration < 0. I.e., the object is decelerating.
Because acceleration already is a rate (##\frac {dv}{dt}## or the second derivative of the position with respect to time). "Rate of acceleration" implies how the acceleration is changing with respect to t.

Like I said earlier, it seems that the part 2 answer scheme wants the student to say simply whether the object is accelerating or decelerating. The simplest answer, without getting into the weeds with pos./neg. acceleration is that the object is accelerating for the first two seconds, and decelerating for the next five seconds. When humans write these questions and answers for automated-scoring tests, they don't always think of all the varied ways that other humans can respond.
Mark, yes, apologies - the straight descending line is between t = 2 and 7.
And yes, concave up for 0 to 2. [The paper is here BTW]
As before, I now understand what you and the question setter meant - rate is measurement of how one thing changes with another. And you used my own example - rate of pay - to show me how. I agree and I have moved my view somewhat - but I continue to thik that "rate of acceleration" should not be clobbered - though I think "rate of change of acceleration" should be.

On to part 2. I don't think the simplest answer is necessarily the one you give, and whether or not it is the simplest answer hasno bearing on whether or not it is the only acceptable one.I agree that people are human - and I'm not out for a linching - I'm just trying to extablishcorrect and incorrect and go from there.

You have helped me to see that question setter's viewpoint on the "rate" argument, though I think it's marginal and the marking scheme is mean. I still think he is totally wrong on part 2.

Anyway, thank you for your time and your patience with me. Before I can form a confident view of OCR's performance I'll need to look at a few more examples and hopefully people here will be as generous as you ave been.
 
Justify means you need to explain how you got the answer. Many students may be able to tell you the resistance is effectively infinite below the threshold voltage, but if you were to ask them why it is so, they'll say, "I don't know," because all they did was memorize a result without understanding the reasoning behind it.

As to what level of justification is appropriate, that's a part of the art of test-taking. Frankly, given the question, I think your interpretation of explaining in terms of semiconductor behavior would be overkill, but you know the test better than I do.
Vela - thanks. "Justify" CAN mean "explain how you got the answer" but that is not the only possible meaning it can have. We could do an etymological analysis (Justify means "show or prove to be right or reasonable") but that doesn't really help us here. One can look at that and say "See - just what I said!" but there are still other valid interpretations.

If you feel this is wrong, I'd appreciate you explaining why it CANNOT mean "explain what is happening in the semiconductor".

Thing is - this could be easily fixed by clarifying the question to something like "by referring to the graph, justify your answer" or "explain your answer in terms of the data shown in the graph".
 
??? that's not what you said.


I think an increasing rate of pay would be when you get a raise, but I sort of see how you are looking at it. You are considering that the PAY is increasing, not that the RATE of pay is increasing.
I agree with you Phinds - whilst Mark's interpretation is strictly correct, I don't think it would be the common interpretation - that would be the pay raise interpretation. Of course, appeaking to "common interpretations" is dodgy ground in the physics world where common parlance is woefully inadequate, but I continue to think that the stipulation to punish "rate of acceleration" is wrong.

And in fact, if rate DOES mean that - then what on earth does "rate of change" actually mean? Or is it tautological?
 
You are correct on both counts and I also agree with your analysis of the other problem you mentioned.

I've often found this kind of idiocy on "standardized" tests, which often seem to have been formulated by bureaucrats, not teachers.
I'm curious to know if you work in teaching Phinds, if you're willing to say.
 

PeroK

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Regarding acceleration, I agree that acceleration could be negative, zero, or positive, but if the question specifically asks whether an object is accelerating or decelerating, and a < 0, then the correct answer would be "decelerating," IMO.
Technically, however, acceleration is a vector. There are no cases, IMO, where acceleration in a physics context is invalid and deceleration valid.

Also, deceleration is not where ##a < 0##. Deceleration is where, in 1D motion, acceleration and velocity have opposite signs.

Or, more generally in 3D motion, where the magnitude of the velocity is decreasing.

For example.

A body falling under gravity is subject to a constant acceleration. There can be no argument about this. This is not an incorrect statement, whatever the OCR exam board thinks.

The idea that the only valid description is that the body decelerates on the way up and accelerates on the way down is not physics.
 
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I'm curious to know if you work in teaching Phinds, if you're willing to say.
No, but my wife does and has shown me some tests, in addition to my own personal experience.

I have some experience in tutoring young people who are studying for their high school equivalency degree (used to be GED but is now called TASC in New York).

Just the other night I was looking at a new teaching manual that they are not yet required to use but are going to be required to use sometime soon and it is HORRIBLE. One of the first problems I looked at was a supposedly simple word problem at the 4th / 5th grade level and it was so confusing that I could not figure it out so I asked the lady there who had been teaching and tutoring math for 30 years and she could not figure it out either. The answer and discussion they gave for the question at the end of the section made no sense at all. She said the whole new set of books is almost that bad throughout. And this is just NY state. I hear others (not all of course) are just as bad or worse
 
No, but my wife does and has shown me some tests, in addition to my own personal experience.

I have some experience in tutoring young people who are studying for their high school equivalency degree (used to be GED but is now called TASC in New York).

Just the other night I was looking at a new teaching manual that they are not yet required to use but are going to be required to use sometime soon and it is HORRIBLE. One of the first problems I looked at was a supposedly simple word problem at the 4th / 5th grade level and it was so confusing that I could not figure it out so I asked the lady there who had been teaching and tutoring math for 30 years and she could not figure it out either. The answer and discussion they gave for the question at the end of the section made no sense at all. She said the whole new set of books is almost that bad throughout. And this is just NY state. I hear others (not all of course) are just as bad or worse
Thanks. That is very depressing. One wonders what kind of an organisation can oversee such stuff. One whic hshould be packed full of intelligent well-motivated professionals, and yet - one which clearly isn't.
 
Technically, however, acceleration is a vector. There are no cases, IMO, where acceleration in a physics context is invalid and deceleration valid.

Also, deceleration is not where ##a < 0##. Deceleration is where, in 1D motion, acceleration and velocity have opposite signs.

Or, more generally in 3D motion, where the magnitude of the velocity is decreasing.

For example.

A body falling under gravity is subject to a constant acceleration. There can be no argument about this. This is not an incorrect statement, whatever the OCR exam board thinks.

The idea that the only valid description is that the body decelerates on the way up and accelerates on the way down is not physics.
Extremely well put.
 

PeroK

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Extremely well put.
PS one further point is that the concept of deceleration is dependent on the reference frame. An aircraft taking off is accelerating relative to the surface of the Earth, but if it is travelling west it is decelerating in an inertial frame where the Earth is rotating west to east.

Again this shows the limited value of the whole concept of deceleration to physics. The acceleration is frame-invariant. Whereas the magnitude and direction of the velocity are not.
 
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PS one further point is that the concept of deceleration is dependent on the reference frame. An aircraft taking off is accelerating relative to the surface of the Earth, but if it is travelling west it is decelerating in an inertial frame where the Earth is rotating west to east.

Again this shows the limited value of the whole concept of deceleration to physics. The acceleration is frame-invariant. Whereas the magnitude and direction of the velocity are not.
Agreed.
 

anorlunda

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I can't think of any solution to this problem that I would not preface with HORRORS.

1 HORRORS no standardized tests. Every teacher invents his own standard.

2 HORRORS every standardized test grader decides for himself which answer is correct.

3 HORRORS the wordings of questions and answer choices is designed by committee.

4, 5, 6, ... there is no end to horrible choices.

Could it be that the underlying evil is the multiple choice question?
 

atyy

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@ChrisXenon, I don't agree with all your points, but definitely quite a number of them. Just wondering - do you have problems with A-levels boards other than OCR? Are the marking schemes consistent across different A-level boards?
 

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